Among liquid crystal display (LCD) devices, reflection-type LCD devices which conduct display by reflecting light which has entered from outside have drawn particular attention since they do not necessitate backlights as light sources, thereby consuming less power and being able to be formed smaller and lighter.
Therefore, in the case where a conventional TN-type or STN-type LCD element is applied to a reflection-type LCD device, the LCD element has to be provided between a pair of polarizers, with a reflection plate provided on its outer side. Therefore, due to the thickness of glass substrates used therein, a parallax occurs in accordance with an angle with which the user views the glass substrates, that is, an angle between a normal direction of the glass substrate and a direction in which the user views the LCD element. As a result, a drawback in that the display is seen dual occurs.
Besides, conventionally, multi-color display or full-color display is realized by, for example, providing a micro color filter of three dots (red, green, and blue) for each pixel in a liquid crystal cell, and conducting additive mixing. However, since the aforementioned liquid crystal display mode necessitates two polarizers, the display is very dark, and on top of that, the additive mixing cannot be adequately carried out due to the occurrence of the parallax, the above multi-color or full-color scheme has not been adopted to the reflection-type color display device.
Then, recently, an LCD element in which one polarizer and one reflection plate are used has been developed. For example, a direct-view LCD device of the reflection type in which a twist-aligned liquid crystal layer is provided between (1) a minutely uneven reflection plate (provided on an inner side of a cell) and (2) a polarizer is disclosed in the Japanese Publication for Laid-Open Patent Application No.3-223715/1991 (Tokukaihei 3-223715).
FIG. 16 is a cross-sectional view illustrating an arrangement of a reflection-type LCD device 100 disclosed in the above publication. The reflection-type LCD device 100 is composed of transparent substrates 101 and 102, a twist-aligned liquid crystal layer 103, transparent electrodes 108, uneven reflective electrodes 105, and alignment films 106 and 107. For example, the twist-aligned liquid crystal layer 103 having a positive dielectric anisotropy is provided between the transparent substrates 101 and 102 made of glass. The alignment films 106 and 107 are formed on the transparent electrodes 108 provided on the transparent substrate 101, and on the uneven reflective electrodes 105 provided on the transparent substrate 102, respectively. Alignment processing, such as a rubbing operation, has been applied to surfaces of the alignment films so that liquid crystal molecules of the liquid crystal layer 103 are aligned in parallel with the substrates. Herein, a twist angle of the liquid crystal is set to, for example, 63.degree. in a state where no voltage is applied.
The display mechanism is explained as follows, with reference to FIGS. 17(a) and 17(b). In a bright state, as illustrated in FIG. 17(b), linearly polarized light which has passed through the polarizer 104 from the transparent substrate 101 side enters the twist-aligned liquid crystal layer 103. The linearly polarized light passes the twist-aligned liquid crystal layer 103 and is converted to linearly polarized light, and is diffusely reflected by the reflection plates 105, then becoming linearly polarized light. Subsequently, by passing through the twist-aligned liquid crystal layer 103, it becomes linearly polarized light having a plane of polarization in the same direction as that when first entering the liquid crystal layer 103, and goes through the polarizer 104 without changing. Thus, the bright display is realized.
On the other hand, as illustrated by FIG. 17(a), when a voltage is applied, the liquid crystal layer 103 loses the twist alignment, and becomes aligned in a direction in accordance with a direction of the electric field. The linearly polarized light having passed through the polarizer 104 from the transparent substrate 101 side enters the liquid crystal layer 103. The linearly polarized light thus having entered passes through the liquid crystal layer 103, becoming right-handed circularly polarized light. The, it becomes left-handed circularly polarized light due to the reflection plates 105, being diffusely reflected, and again enters the liquid crystal layer 103. Here, the left-handed circularly polarized light becomes linearly polarized light tilted through an angle of 90.degree. as compared with the light when first entering the liquid crystal layer 103, and is absorbed by the polarizer 104. Thus, the dark display is realized. With this arrangement, no parallax occurs and a refractive index is higher, as compared with the arrangement wherein two polarizers are used.
Furthermore, a reflection-type LCD device wherein one polarizer, a quarter wave length plate, polymer dispersion liquid crystal, and mirror reflection plates are provided in this order from a light incidence side is disclosed in the Japanese Publication for Laid-Open Patent Application No. 7-28054/1995 (Tokukaihei 7-28054).
The reflection-type LCD device is characterized in that the polymer dispersion liquid crystal layer is sealed between transparent electrodes provided on one side surface of a first transparent substrate and reflection electrodes provided on a surface of a second transparent substrate, and the quarter wave length plate and a polarizer are laminated on the other side surface of the first transparent substrate. The liquid crystal of the polymer dispersion liquid crystal layer is aligned at random when no voltage is applied, and the polarization of light is cancelled when the light passes therethrough, thereby causing the display to shift to a bright state. On the other hand, the liquid crystal is aligned perpendicularly with respect to the transparent electrodes and the reflection electrodes when a voltage is applied. Therefore, a birefringence effect does not occur with respect to the perpendicular incident light. For this reason, light which enters from outside and then becomes about to again go to outside passes the quarter wave length plate twice, and hence comes to have a phase shift substantially corresponding to half wave, thereby having its plane of polarization rotated through an angle of 90.degree.. As a result, the light is absorbed by the polarizer when the light is outgoing, thereby causing the display to shift to a dark state.
However, according to the reflection-type LCD device disclosed in Tokukaihei 3-223715, to realize a white display, a diffusing reflection plate is indispensable behind the liquid crystal layer, since the display of the device is conducted by controlling the transmission state and the absorption state of the liquid crystal element with the use of a voltage. In other words, the performance of the diffusing reflection plate determines display performance. For example, the reflection plate used for the display scheme described above does not cause the incident light to maintain polarization, the conversion of the right-handed circularly polarized light to the left-handed circularly polarized light, or the reverse conversion is not efficiently carried out, and light is leaked during the dark display, thereby impairing contrast.
Furthermore, as the reflection member having a property of maintaining polarization, a flat mirror reflection member can be used, but since objects outside are reflected on its surface as they are, neighboring information is also reflected and is mixed in the display when it is in the bright state (this phenomenon is hereinafter referred to as "mixing reflection"), thereby impairing the visibility. Therefore, the reflection plate preferably has a light diffusing property.
Therefore, as a method for forming a reflection plate which is capable of controlling the polarizing property while maintaining the light diffusing property, a method whereby an aluminum film is formed on an uneven part made of a smooth photosensitive resin is disclosed in the Japanese Publication for Laid-Open Patent Application No. 7-218906/1995 (Tokukaihei 7-218906). It is reported in the publication that a degree of maintaining the polarizing property (hereinafter referred to as polarizing property maintenance degree) has to be not less than 50 percent so as to achieve a contrast degree of 4, while the polarizing property maintenance degree has to be not less than 70 percent so as to achieve a contrast degree of 7. As the polarizing property maintenance degree is enhanced, the contrast degree becomes higher, but an angle of visibility becomes narrower, thereby causing a drawback in that the display becomes hard to see. On the other hand, as the polarization maintenance degree is lowered so as to enhance the diffusing property and widen the angle of visibility, the contrast is drastically impaired. In short, it is impossible to provide a reflection plate which perfectly maintains the polarizing property and the light diffusing property both, and a monochromatic display which is easy to see cannot be realized.
Besides, as to the reflection-type display device disclosed in Tokukaihei 7-28054, the dark state without diffusing is realized. However, in the bright state, it has a problem as follows. Half of incident light is absorbed by the polarizer, and then, the light is further decreased to 1/2 by the polarizer when the light goes out, since the light is directed at random and the polarization thereof is cancelled. As a result, the brightness becomes 1/4 or less, making the display dark.